U.S. patent number 4,481,779 [Application Number 06/506,811] was granted by the patent office on 1984-11-13 for cryogenic storage container.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Alfred Barthel.
United States Patent |
4,481,779 |
Barthel |
November 13, 1984 |
Cryogenic storage container
Abstract
A storage container for shipping transportable materials at
cryogenic temperatures including a vessel which opens to the
atmosphere and contains a micro-fibrous structure for holding a
liquified gas such as liquid nitrogen in adsorption and capillary
suspension. The micro-fibrous structure comprises a core permeable
to liquid and gaseous nitrogen and an adsorption matrix composed of
a web of inorganic fibers surrounding the core in a multi-layered
arrangement.
Inventors: |
Barthel; Alfred (Indianapolis,
IN) |
Assignee: |
Union Carbide Corporation
(Danbury, CT)
|
Family
ID: |
26092297 |
Appl.
No.: |
06/506,811 |
Filed: |
June 22, 1983 |
Current U.S.
Class: |
62/46.3;
220/560.12; 220/592.26; 220/592.27; 220/901 |
Current CPC
Class: |
F17C
3/08 (20130101); F17C 11/00 (20130101); Y10S
220/901 (20130101); F17C 2203/018 (20130101) |
Current International
Class: |
F17C
3/08 (20060101); F17C 11/00 (20060101); F17C
3/00 (20060101); F17C 011/00 () |
Field of
Search: |
;62/45,48,514R,385
;220/420,421,422,423,424,425 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Johns-Manville-Insulation Product Literature-1. -Scored Block
Insulation, 12/1980. .
2. -Pipe & Block Insulation, 12/1980..
|
Primary Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Lieberstein; Eugene
Claims
I claim:
1. An open-to-atmosphere storage container for transporting
materials at cryogenic temperatures having a micro-fibrous
structure adapted for holding a liquified gas such as liquid
nitrogen in adsorption and capillary suspension within the interior
of the container, said micro-fibrous structure comprising a core
permeable to gaseous and/or to liquid nitrogen, with said core
being centrally disposed in said container and having at least one
void adapted for the removable placement of the transportable
materials; and a liquified gas adsorption matrix composed of a web
of very small diameter inorganic fibers surrounding said core in a
multilayered arrangement having an outside diameter conforming to
the inside diameter of the storage container.
2. An open-to-atmosphere storage container as claimed in claim 1
further comprising an inner vessel containing said micro-fibrous
structure, said inner vessel having an end open to the atmosphere,
an outer shell surrounding said inner vessel and spaced apart
therefrom to define an evacuable space therebetween for forming a
vacuum upon being evacuated, insulation material occupying said
evacuable space and means for sealing the evacuated space between
the outer shell and the inner vessel.
3. A storage container as claimed in claim 2 further comprising a
neck tube extending between the open end of the inner vessel and
the outer shell, said neck tube being open to the atmosphere, and a
neck tube core loosely fitting within the open neck tube for
assuring open communication between the atmosphere and the inner
vessel.
4. An open to atmosphere storage container as claimed in claim 3
wherein said insulation material is composite multilayered
insulation composed of a radiant heat reflecting component and a
low heat conducting component disposed in relation to the radiant
heat reflecting component so as to minimize the transfer of heat
across evacuable space.
5. An open to atmosphere storage container as claimed in claim 2
wherein said insulation material consists essentially of finely
divided particles of agglomerate sizes, less than about 420
microns, of low heat conducting substances such as perlite,
alumina, and magnesia, with or without admixture of finely divided
radiant heat reflecting bodies having reflecting metallic surfaces
of sizes less than about 500 microns.
6. A storage container as claimed in claims 1, 3, or 4 wherein said
multi-layered structure surrounding said core is in the form of a
coiled roll or cylindrical shape having multiple layers of said
inorganic fiber web in relatively compact engagement with one
another.
7. A storage container as claimed in claim 6 wherein the diameter
of said inorganic fibers range between 0.03 to 8 microns.
8. A storage container as defined in claim 7 wherein said inorganic
fibers are composed of borosilicate glass.
9. A storage container as defined in claim 7 wherein said inorganic
fibers are composed of quartz.
10. An open to atmosphere storage container as claimed in claim 8
wherein said core is of a hollow tubular construction with said
void defined by the hollow space in said core.
11. A storage container as defined in claim 10 wherein said core
has a multiple number of small perforated openings of a suitable
geometric configuration and size.
12. A storage container as defined in claim 10 wherein said core is
of an intrinsically permeable structure having inherent
micro-passages throughout its body.
13. A storage container for shipping transportable materials at
cryogenic temperatures comprising:
an inner vessel having an open end; an outer shell having an open
end; access means connecting said open end of said outer shell to
said open end of said inner vessel such that said inner vessel is
suspended from said outer shell in a spaced apart relationship for
defining an evacuable space therebetween; insulation means disposed
within said evacuable space; and a micro-fibrous structure located
within said inner vessel for holding liquid nitrogen by adsorption
and capillary suspension, said micro-fibrous structure comprising a
gas permeable core having a void disposed in said inner vessel in
alignment with said access means, with said access means providing
ingress and egress to said void for removably inserting said
transportable materials and a liquid nitrogen adsorption matrix
composed of a web of very small diameter inorganic fibers
surrounding said core in a multi-layered arrangement with an
outside diameter conforming to the inside diameter of said inner
vessel.
14. A storage container as defined in claim 13 wherein said
multi-layered arrangement is in the form of a coiled roll.
15. A storage container as defined in claim 13 wherein said
multi-layered arrangement is a stack of superimposed disks with
each disk formed from said web.
Description
This invention relates to open to atmosphere storage containers for
storing bio-systems at cryogenic temperatures and more particularly
to an open to atmosphere shipping container adapted to hold a
supply of liquid nitrogen for refrigerating a stored biological
product during transportation from one location to another over a
relatively long time period.
BACKGROUND OF THIS INVENTION
The shipment of heat-sensitive bio-systems, as for instance semen,
vaccines, cultures of bacteria and viruses at optimal temperature
levels between about 78 K. and 100 K., poses a series of
difficulties. The vials or "straws", in which the biologicals are
hermetically sealed, must be kept continuously at near liquid
nitrogen temperature to preserve the viability of the biological
product. But since the boiling point of liquid nitrogen at ambient
pressure is 77.4 K. (-320.4.degree. F.) the cryogen holding vessel
(refrigerator) must remain open to the atmosphere to vent the
boiled-off gas and thus avoid a dangerous pressure build-up inside.
For this reason open-to-atmosphere liquid nitrogen vessels are used
for refrigeration. It is obvious that such vessels must be kept
upright at all times to prevent spillage of the cryogen. This
condition is difficult to control during a long shipment unless an
attendant accompanies the vessel on the trip which is rarely a
feasible option.
To overcome the difficulties associated with the shipment of
biologicals at cryogenic temperature a shipping container was
developed in which the liquid nitrogen is retained in a solid
porous mass by adsorption, capillarity and absorption. Based upon
this development a patent issued to R. F. O'Connell et al. in 1966
as U.S. Pat. No. 3,238,002. The shipping container described in
this patent is of a double-walled construction to provide a vacuum
space around the inner vessel which holds the liquid nitrogen. The
vacuum space is filled with a multilayer insulation to reduce heat
transfer by radiation. An adsorbent and a getter are part of the
system to maintain vacuum integrity. The inner vessel is filled
with the solid porous mass which, when saturated with liquid
nitrogen, will hold the cryogen by adsorption, and capillarity as
well as by absorption, similar to a sponge "holding" water. In the
center of the porous filler core one or more voids are provided to
hold the vials containing the biologicals.
The solid components of the porous mass described in U.S. Pat. No.
3,238,003 are silica (sand), quick-lime, and a small amount of
inert heat resistant mineral fibers such as asbestos. The porous
mass is formed starting with an aqueous slurry of the filler
components which is poured into a mold and then baked in an
autoclave under precisely controlled equilibrium conditions of
pressure and temperature. The components undergo a chemical
reaction forming a porous mass of calcium silicates, reinforced by
inert fibers. The evaported water leaves inside the dried out solid
structure microscopic voids, of complex geometry, sometimes
referred to as "pores", which comprise on the average 89.5% of the
apparent solid volume. Since the resulting mass is incompressible
the mold must either provide the mass with a shape conforming to
the inner vessel of the storage container or it must be machined to
size. The porous mass is filled with liquid nitrogen by submerging
it in a liquid nitrogen bath until it is saturated. The filling
operation for a conventional two liter container housing a
sand-lime porous mass matrix takes about twenty-four hours.
The baked sand-lime porous mass is intrinsically hydrophilic.
Because of this property moisture must be periodically driven out
of the porous mass matrix to prevent the accumulation of trapped
water. If this is not done, the trapped water will turn into ice
crystals every time it is exposed to liquid nitrogen and eventually
will crack the brittle microstructure of the filler. This may be
prevented by periodically heating the porous structure to above
100.degree. C. after several fill and warm up cycles.
Although the ingredients used in manufacturing the sand-lime porous
mass are relatively inexpensive (deionized water, sand, quick-lime
and inert fibers, as for example asbestos) the finishing operations
in handling a solid porous mass are very expensive due to the high
labor costs involved and the elaborate safety precaustions
required. It is not economically feasible to cast the porous filler
in a cryogenic holding vessel. Elaborate safety precautions are
indispensable when handling substances like asbestos fibers and
noxious dust. In addition, the thermal energy cost is very high for
the manufacturing process of the sand-lime filler mass.
Alternative systems for retaining liquid nitrogen in a storage
container through a combination of adsorption, absorption and
capillarity have in the past being investigated by those skilled in
the art. The use of high porosity blocks, artificial stones, bricks
and light papers made from cellulose fibers such as towels and
bathroom tissues have been studied and, in general have been
dismissed as inferior compared to the use of the sand-lime porous
mass matrix due primarily to their low porosity. The average
porosity of the sand-lime porous matrix is 89.5% whereas the
porosity of a matrix fabricated from any of the aforementioned
materials is below 60%. More recently block insulation material
composed of hydrous calcium silicate has been used as the
adsorption matrix. Such material is closer in porosity to the
sand-lime porous mass composition but also has most of the
shortcomings of the sand-lime porous mass composition. The porosity
of the filler matrix determines for a given size shipping container
its liquid nitrogen capacity. The porosity and rate of evaporation
are the most important characteristics of a liquid nitrogen storage
container for transporting a product at cryogenic temperatures. A
storage container using a sand-lime porous mass matrix has an
average 5 day holding time based on an evaporation rate of 0.33
liters per day and a liquid capacity of 1.6 liters.
Accordingly, the art has long sought a less expensive and much more
efficient liquid nitrogen adsorption system as an alternative to
the storage systems in present use.
OBJECTS OF THE INVENTION
It is therefore, the principle object of the present invention to
provide a low cost refrigerated storage container for transporting
bio-systems at cryogenic temperatures.
It is another object of the present invention to provide a
refrigerated storage container for shipping a bio-system over a
long holding period during which time the bio-system is sustained
in suspended animation at cryogenic temperatures.
It is yet another object of the present invention to provide a low
cost refrigerated storage container having a liquid nitrogen
adsorption matrix which has a high average holding capacity and is
intrinsically hydrophobic.
A still further object of the present invention is to provide a
refrigerated storage container having a liquid nitrogen adsorption
matrix which has a higher adsorptivity than state of the art liquid
nitrogen adsorption matrices and which will fill to capacity in a
substantially reduced time period.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the present invention will become
apparent from the following detailed description of the invention
when read in conjunction with the accompanying drawings of
which:
FIG. 1 is a front elevational view, in section, of the storage
container of the present invention; and
FIG. 2 is a perspective view of the nitrogen adsorption structure
of FIG. 1 cut lengthwise in half.
SUMMARY OF THE INVENTION
The storage container of the present invention includes a vessel
which opens to the atmosphere and contains a micro fibrous
structure for holding a liquified gas such as liquid nitrogen in
adsorption and capillary suspension. The microfibrous structure
broadly comprises a core permeable to liquid and gaseous nitrogen
having a cavity extending therethrough which is adapted for the
removable placement of a product to be transported at cryogenic
temperatures and a liquid nitrogen adsorption matrix composed of a
web of inorganic fibers of e.g. glass or quartz or a ceramic of
very small diameters surrounding the core in a multilayered
arrangement preferably in the form of a coiled roll having a
multiplicity of layers and an outside diameter conforming to the
inside diameter of the vessel. The core is preferably tubular with
the hollow center used as the storage cavity for receiving the
transportable product. The storage container is preferably of a
double walled construction to provide a vacuum space between the
inner and outer walls with the inner wall defining the liquid
nitrogen holding vessel. The vacuum space is filled with insulation
preferably multilayer insulation consisting of e.g. low emissivity
radiation barriers interleaved with low heat conducting
spacers.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is illustrated in the preferred embodiment of FIG. 1
which shows a storage container 10 having a self supporting outer
shell 12 surrounding an inner vessel 13. The inner vessel 13 is
suspended from the outer shell 12 by a neck tube 14. The neck tube
14 connects the open neck 15 of the inner vessel 13 to the open
neck 16 of the outer shell 12 and defines an evacuable space 17
separating the outer shell 12 and the inner vessel 13. A neck tube
core 18 is removably inserted into the neck tube 14 to reduce heat
radiation losses through the neck tube 14 as well as to prevent
foreign matter from entering into the inner vessel 13 and to
preclude moisture vapors from building up highly objectionable
frost and ice barriers inside the neck tube 14. The neck tube core
18 should fit loosely within the neck tube 14 to provide sufficient
clearance space between the neck tube 14 and the neck tube core 18
for assuring open communication between the atmosphere and the
inner vessel 13.
The evacuable space 17 is filled with insulation material 19
preferably composed of low emissivity radiation barriers, like
aluminum foil, interleaved with low heat conducting spacers or
metal coated nonmetallic flexible plastic sheets which can be used
without spacers. Typical multilayer insulation systems are taught
in U.S. Pat. Nos.: 3,009,600, 3,018,016, 3,265,236, and 4,055,268,
the disclosures of which are all herein incorporated by reference.
A plurality of frustoconical metal cones 20 may be placed around
the neck tube 14 in a spaced apart relationship during the wrapping
of the insulation in order to improve the overall heat exchange
performance of the storage container 10 following the teaching of
U.S. Pat. No. 3,341,052 the disclosure of which is herein
incorporated by reference.
To achieve the required initial vacuum condition in the evacuable
space 17, the air in the evacuable space 17 is pumped out through a
conventional evacuation spud 21 using a conventional pumping system
not shown. After the evacuation has been completed the spud 21 is
hermetically sealed under vacuum in a manner well known in the art
using, for example, a sealing plug and cap (not shown).
An adsorbent 22 is located in the vacuum space 17 to maintain a low
absolute pressure of typically less then 1.times.10--.sup.4 torr.
The adsorbent 22 may be placed in a retainer 23 formed between the
shoulder 24 and the neck 15 of the inner vessel 13. The retainer 23
has a sealable opening 25 through which the adsorbent 22 is
inserted. The adsorbent 22 is typically an activated charcoal or a
zeolite such as Linde 5A which is available from the Union Carbide
Corporation. A hydrogen getter 26 such as palladium oxide (PdO) or
silver zeolite may also be included in the vacuum space 17 for
removing residual hydrogen molecules. To those skilled in the art
it is apparent that other locations, as well as methods of
placement of the adsorbent and the hydrogen getter, are
feasible.
The inner vessel 13 contains a micro-fibrous structure 27 for
holding liquid nitrogen by adsorption and capillary suspension. The
micro-fibrous structure 27, which is shown in partial perspective
in FIG. 2, comprises a core 28 and a glass fiber matrix 30 composed
of a continuous web of glass fibers surrounding the core 28 in the
form of a coiled roll which is preferably cylindrical in
configuration. Although one does not ordinarily associate glass
with characteristics such as sponginess and porosity, it has been
discovered in accordance with the present invention that reasonably
compacted webs of glass fibers possess high capacity for holding
liquid nitrogen by adsorption and capillary suspension provided the
glass fibers in forming the web are of very small diameter and
provided further that the adjacent web layers made of glass fibers,
are coiled up into a roll with a reasonable degree of compactness
between the aggregate layers of the roll. The coiled up roll was
found to be preferred in constructing the micro fibrous structure
27 of this invention. However, to those skilled in the art it is
apparent that an alternate micro-fibrous matrix configuration can
be formed by cutting out a multiplicity of individual discs from a
web of micro fiberglass, punching a hole in the center of each disk
and stacking up the disks about the core into a relatively compact
body having an external configuration as shown in FIG. 2. The
liquid nitrogen is held in the coiled-up micro-fibrous matrix by
molecular adsorption to the enormous aggregate area of the micro
fibers, as well as by capillary suspension made possible by the
microscopic intra-fibrous voids between individual fibers. It is
therefore of importance that the diameters of the glass fibers be
as small as possible with the preferred range from 0.03 to 8
microns.
The web of glass fibers should preferably be formed without using
any ridgidizing binders or cements. Substantially binderless
inorganic fiber webs are commercially available from e.g., the
Dexter Corporation in Windsor Locks, Conn. under the present
material description designation of Grade 233; from Manning Paper
Company, Troy, N.Y., web 9# Manninglas 1000 with a mean glass fiber
diameter of 0.63 micron; webs from Pallflex Products Corporation,
Putnam, Conn., under the designation of Tissuglas 60A, Tissuglas
100A, and Tissuquartz. The example Grade 233 web of glass fibers
used in this invention are composed of borosilicate glass with the
glass fibers ranging from 0.5 to 0.75 microns in diameter. The
non-woven web is made in a fashion similar to that used in the
paper making process. The glass fibers are put into an aqueous
suspension to form a mesh which is applied to a moving screen,
dried out, compressed and compacted into a continuous web of glass
fibers having a felt like consistency, wherein the strutural
stability is effected primarily by intra-fibrous friction.
The core 28 is preferably of tubular geometry having a central void
31 into which the biological product is to be placed during
shipment. The core 28 can be of any material composition, e.g.,
metal or plastic that will remain structurally stable and retain
its form after being repeatedly subjected to cold shocks at liquid
nitrogen temperatures. To maintain the lowest possible temperature
within the cavity 31 the core 28 must be permeable to the nitrogen
gas that boils off from the liquid nitrogen stored in the glass
fiber matrix 30. The permeability of the core can be provided by
forming the core 28 from a perforated sheet rolled into a tube or
using a porous sintered tube without apparent holes. Where
perforations are used, the holes 29 in the wall of the core 28 must
be small enough to prevent any loose fiber particles from passing
across the core wall 28 into the storage cavity 31 containing the
biological product. Hole sizes of 1 millimeter in diameter have
been found to be adequate for this purpose.
The matrix 30 is preferably formed by winding a continuous web of
glass fibers around the core 28 under reasonably high tension to
assure a sufficient degree of compactness between all of the layers
in the finished roll. This is readily established by forming the
matrix 30 with about 200 to 280 layers per radial inch of roll
thickness. The outside diameter of the glass fiber web matrix 30
should conform to the inside diameter 11 of the inner vessel
13.
The storage container 10 of FIG. 1 is preferably assembled starting
with an inner vessel 13 of a two piece construction having an upper
cylindrical section 32 with an open end bottom 34 and a lower
section 33. The micro-fibrous structure 27 is inserted into the
upper section 32 through its open bottom 34 before the lower
section 33 is attached. The upper section 32 is crimped around the
open bottom 34 to facilitate attachment of the lower section 33.
The two sections 32 and 33 of the inner vessel 13 may be joined by
welding the mated ends around the crimped edge at the bottom 34 of
the upper section 32 to form a unitary structure which encloses the
micro-fibrous structure 27. The core 28 of the micro-fibrous
structure 27 is substantially aligned with the open neck 15 of the
inner vessel 13 and should be disposed in substantially coaxial
alignment with the neck tube 14. The neck tube 14 can be joined to
the open neck 15 of the inner vessel 13 and to the open neck 16 of
the outer shell 12 by a variety of means, such means depending
primarily on the materials of the two constituents of a particular
joint.
The outer shell 12 is also of a two piece construction with an
upper cylindrical section 35 and a lower bottom section 36. The
inner vessel 13 is inserted into the upper section 35 before the
two sections are joined to each other. Where a wrapped composite
insulation system is used, the inner vessel is first wrapped with
the layers of insulation preferably using the heat exchange cones
20 before the inner vessel 13 is inserted into the upper section
35. The adsorbent 22 and getter composition 26 may be added at this
time. The upper section 35 may have a crimped end 37 to facilitate
attachment of the lower section 36. The two sections 35 and 36 are
then welded together to form a unitary structure. Instead of
circumferential crimping as shown in 34 and 37 of FIG. 1 other
means of alignment of mating cylindrical components can be used,
e.g. butt welding with a back-up ring or tack welding in a jig.
Four prototypes, designated for identification purposes as 2DS
units, were built under normal manufacturing conditions in
accordance with the preceeding description.
The liquid capacity of the glass fiber web matrix was determined by
the apparent volume of the matrix and its porosity. The design
volume of the prototype matrix was 2,370 cm.sup.3. The porosity of
the fibrous adsorption medium of this invention was found
experimentally to vary between 89.4% and 95.8%. The calculated mean
value of the porosity was 92%. The mean liquid capacity of the
prototype matrix was therefore: 2,370 cm.sup.3 .times.0.92=2,180
cm.sup.3 or 2.18 liters.
This then was the design figure for the amount of liquid nitrogen
to be held within the fibrous matrix by adsorption and capillarity
without drainage or spillage. Actual test data showed these figures
to be remarkably close.
In service, the liquid nitrogen, held in the matrix, keeps
evaporating due to the unavoidable heat inflow from ambient
resulting from the temperature gradient between ambient and liquid
nitrogen. Eventually all the cryogen is bound to boil off
completely, leaving the storage compartment for the temperature
sensitive product without refrigeration. Considering this
circumstance, which in essence is a race between the holding time
of the storage container and the shipping time of the product, the
rate of evaporation is the most important characteristic of a
shipper-refrigerator.
The evaporation rates of the 4 prototypes of this invention ranged
between 0.088 liter/day and 0.081 liter/day with a mean of 0.083
liter/day. This remarkably low evaporation rate makes it possible
to achieve a mean holding time of ##EQU1## compared to 5 days for
state-of-the-art shippers.
To test the performance of the four 2DS prototypes they were filled
to capacity with liquid nitrogen and left standing for a few days
to cool down and to reach steady state condition in heat transfer.
The necktube was closed with the loosely fitting necktube core 18,
made of a low heat conducting foam composite. The core remained
inside the necktube for the entire duration of the tests. The
gaseous nitrogen, continuously boiling-off from the liquified gas,
had always free passage to atmosphere through the clearance space
between the outside of the loosely fitting core and the inside of
the necktube. Following cooldown the 2DS prototypes were emptied of
all the free flowing liquid nitrogen by turning them upside down.
The units were left in "dry" condition. The only liquid nitrogen
left in the inner container was that which had been adsorbed by the
glass fiber web matrix.
After the nitrogen has been dumped, the weight of each unit was
recorded. The difference in weight between the empty unit with core
(which had been determined before the test) and the weight of the
unit, emptied of all its free flowing nitrogen, determined the
amount of liquid nitrogen adsorbed in the matrix. During the
following 6 days the units were left undisturbed in the test room.
Then the final weight of each unit was taken. The difference
between the last two scale figures determined the amount of liquid
nitrogen boiled off in 6 days from the adsorbed reserve in the
matrix.
The performance of a cryogenic container can be expressed in terms
of holding time or in terms of normal evaporation rate. Both are
being used interchangeably. The normal evaporation rate (NER),
expressed in any convenient mass or volume units of the cryogen per
day, is determined by dividing the weight of the cryogen,
evaporated within a reasonable number of days, by the said number
of days. The relevant data of the tests are summarized in the
following Table I.
TABLE I ______________________________________ 2DS Prototype Number
Data Identification 1 2 3 4 Mean
______________________________________ Wt gms of empty 2DS 2910
2910 2915 2906 2910 (Plus necktube core) Wt gms of 2DS with 4672
4672 4672 4676 4673 adsorbed nitrogen Wt gms of adsorbed 1762 1762
1757 1770 1763 liquid nitrogen only Vol lts of adsorbed 2.18 2.18
2.174 2.19 2.18 liquid nitrogen only Wt. gms of 2DS after 4245 4268
4281 4281 4269 6 days of NER testing Wt gms of evaporated 427 404
391 395 404 nitrogen in 6 days Vol lts of evaporated 0.528 0.5
0.484 0.489 0.5 nitrogen in 6 days Mean NER of 2DS 0.088 0.083
0.081 0.082 0.083 in liters/day Number of days of 24.77 26.26 26.84
26.71 26.14 projected holding time
______________________________________
A test was conducted to establish the absorption rate and filling
time of a micro-fibrous structure for use in a typical storage
container, with the micro-fibrous structure having the following
specification:
______________________________________ Diameter of core (perforated
stainless 4.57 cm steel) Diameter of rolled Dexter Grade 233 14.27
cm glass fiber matrix Height (from bottom to top of matrix) 17 cm
Volume of matrix exclusive of core 2440 cm.sup.3 Weight of empty
structure 1.159 lb. (matrix and core) Weight of the structure
saturated 5.320 lb. with liquid nitrogen (two hours after being
submerged in liquid nitrogen) Weight of liquid nitrogen adsorbed
4.161 lb. in two hours Liquid nitrogen saturated structure 5.320
lb. reweighed after 3 hours Liquid nitrogen saturated structure
5.320 lb. reweighed after 4 hours Fill time for matrix 2 hrs.
Porosity of matrix 95.8% ______________________________________
The invention as described in accordance with the preferred
embodiment should not be construed as limited to a specific
configuration for the core and adsorption matrix in defining the
micro-fibrous structure. For example the core may have a plurality
of voids defined, for example, within a tubular framework with the
voids separated by partitions extending from a solid control post
to the outer tubular wall of the core. In such case only the outer
tubular wall of the core must be permeable to gaseous nitrogen.
* * * * *